The invention has application in the treatment of all classes of patients who require ventilation. The most difficult among those classes to treat is the new-born and, among the new-borns, the premature infant suffering from hyaline membrance disease, a condition in which the lungs are stiff and resist inflation. Lung capacity in such an infant may be no more than five cubic centimeters. In some cases the prescribed respiration rate is as high as 150 and even 180 cycles per minute. To provide a method and apparatus which is both suited to such an application and which is feasible in practice has proven to be a difficult problem.
The difficulty arises from the fact that the care needs of individual infants may differ widely both in the matter of ventilation and in respect of treatment for other conditions. The difficulty arises also from the fact that it is not possible to standardize the physical environment in which the infact and the respiratory apparatus are disposed. In practice, the distance from infant to ventilator is not uniform, even in the same hospital, and ambient temperatures differ and, of course, the characteristics of the commercially available ventilators and humidifiers and heaters and incubators and other enclosures vary greatly. An improvement that is to provide a real and available benefit to infants requiring ventilation must be universal in the sense that it must be compatible with a wide range of physical environments.
The invention utilizes, or is used in conjunction with, any of the commercially available ventilators and tracheal tubes. Almost all tracheal tubes are single lumen tubes. Respiratory gas is inspired and expired along the same passageway. No attempt is made to connect the tracheal tube alternately to a supply circuit and an exhaust circuit. A less dangerous practice is to control respiration by controlling pressure in a "respiratory" space to which the tracheal tube's supply end is exposed. Most of the available ventilators are equipped to supply pressurized respiratory gas to that "respiratory" space and to increase its pressure, and they are equipped to reduce its pressure by withdrawing gas or by permitting gas to escape from that space. Control of pressure reduction is accomplished at the ventilator to ensure control of the frequency of pressure change, and, therefore, respiration rate, and to ensure control of the magnitude of pressure and pressure change. To facilitate such pressure control, it is common to measure pressure at said space at the opening of the tracheal tube and to adjust supply pressure, flow rate, and removal rate at the ventilator.
Gas is very compressible, and the consequence of that quality and of viscous friction along the gas flowpath is to make it difficult to alter pressure rapidly in the respiratory space. The degree of difficulty increases with the volume and hence length of the flowpath from the ventilator to the respiratory space and back to the ventilator, and it increases with respiration rate. In any given instance, the flow resistance and volume can be reduced by shortening the flowpath. A nurse or respiratory therapist does that by selecting the shortest practical length of the supply and exhaust conduits. That having been done, volume can be reduced by selecting conduits of smaller cross-sectional area, but that increases flow resistance. Conversely, selecting a larger conduit reduces resistance but increases volume.
Volume is important because it and compressibility determine compliance, and compliance is manifested out of time phase with pressure whereas resistance is manifested in phase with pressure. The result is that it becomes impossible in a practical system to reduce pressure to atmospheric pressure at an intermediate point along the gas flowpath when the pressure is to be cycled at a relatively high rate. That translates into inability to achieve maximum practical exhalation in ventaliting patients.
The difficulty can be overcome in part by decreasing flow conduit size to reduce volume and compliance. That increases resistance, but the effect of increased resistance is overcome by the application of higher pressure to achieve higher flow rates. However, the increased pressure can result in dangerously high pressure peaks at the respiration point. An alternative is to apply suction at the exhaust end of the flow circuit. That can be dangerous, too. Negative pressure at the respiration point can result in collapsed lungs and, when suction is employed, it is usually machine controlled, manually adjustable and continually monitored.
The invention provides a very practical and effective solution to these problems.